Process of coating paper



Nov. 28, 1950 w, BARRETT ET AL 7 2,532,140

PROCESS OF COATING PAPER Filed Nov. 24, 1943 Fig-i Q 23 Q Q \2 ,r l c 2726 a 25 g 22 0 23d a '0 \7 22a l3 9 28a a |9a 30 30a g 44 F R fZ 39InvenEor-s: William H.Bc1r'r-etf After-neg.

Bernard KAsc lelL Patented Nov. 28, 1950 PROCESS OF COATING PAPERWilliam H. Barrett and Bernard K. Asdell, In-

ternational Falls, Minn., assignors to Minnesota and Ontario PaperCompany, Minneapolis,

Minn.

Application November 24, 1943 Serial No. 511,754

This invention is concerned with a papercOating process to produce acoating having a superior surface in respect to flatness, smoothness,freedom from imperfections, etc. The raw paper stock, in web form, maybe of any suitable kind, and the ingredients contained in the coatingapplied thereto may be chosen from a number of different materials. Thecoating mixture may include for example, a mineral pigment, such asclay, calcium carbonate, blanc fixe, zinc sulphide, titanium dioxide,and the like, and a binder of starch, protein, casein, glue or othersuitable material, all contained in an aqueous dispersion.

In the manufacture of paper for use as high grade printing paper, it isa customary procedure to apply to a paper web (base-stock) a 'layerorlayers of an aqueous suspension of mineral pigment and adhesive,generally organic adhesive; the web is then dried and finished in anymanner desired. Paper so'made is generally referred to asmineral coatedpaper.

The coatingof paper with suspensions of mineral pigments has presentedserious problems for the reasons that such suspensions commonly used "inthe paper art do notof themselves smooth out when applied with evenslight irregularities. Heretofore when the paper wascoated by a directapplication of coating liquid there was formed, unless specialprecautions were taken, ripples or irregularities of the coatingmaterial on the paper which, when dried, produce streaks. Therefore, inmuch of the prior art practice, provision is made for smoothing out theripples or other irregularities formed upon the application, either bythe use of brushes, doctor blade or of smoothing or traveling rolls.Such aids, however, have the drawback that they are cumbersome and areresponsible for acomparatively low rate of production.

In view-of these di'fliculties, the present process proposes to apply apredetermined or measured amount of coating material upon the travelingweb of paper to be coated in such manner as to produce an even coatinginthe first instance and thus obviate the use of subsequent smoothingdevices.

It is one of the principal objects of this invention to provide a methodwhich will produce a coating of'mineral or other pigments having in ahigh degree the qualities of smoothness, and uniformity, andLin-addition, having the advantanges of high output and reliability ofoperatime.

Another object of the invention relates to the 5 Claims. (Cl. 117-403)use oi a coating color having high solid contents to a fast moving webof paper: In general the finer the particle size of the mineral pigment,the better the color and finishing qualities. Clays, for example, fromdifferent sources even in the same particle size group vary in degree ofsurface activity. standpoint of high speed coating operations because itafiects'the degree of hydration in water, starch and the like slurries,and is a controlling factor in flow properties which a given coatingcomposition will have.

All starch-mineral coating colors show thixotropic properties. With anygiven mineral coating composition these properties become morepronounced with increasing solids contents. In order to avoid confusingthe terms used in describing coating colors flow properties arehereinafter defined:

Thixotropy is a reversible cso'lgel structure shown by many colloidalsuspensions of high solid content. When a thixotropi'c solution isagitated, the absolute viscosity becomes lower as the speed ofagitationincreases. When theagitation ceases the solution will againgradually set to a'flrm jell.

Dilatancy is the opposite of thixotropy. As the agitation speedincreases, the absolute viscosity increases. Unlike thixotropy, no timefactor is involved in dilatancy.

Viscosity .is the internal resistance of a fluid to flow. The absoluteunit of viscosity is poise;

Fluid-ity is the reciprocal of viscosity.

Plasticity is the rate of decrease of viscosity with increasing rates offlow. A true plastic shows a definite yield point which is not :the casewith most coated material. The term pseudoplastic is sometimes used.

Another important phenomenon of trixotropic solutions is that as theviscosity of the solution decreases by agitation, the rate at which thesolution'will. give 11p its water to an absorbingunedium such as a web'of paper increases. This is a very important factor in paper coatin for:it determines the rate at which the coating solution will wet the paperat the moment of contacuantl this in turn influences the character ofthe coat ing film. For example, it has been found in-our coating'processthat if the coatinggives up water to the paper "fast enough so that ithas become set (that is it is no longer a fluid film) before leaving thenip of the applicator roll, 'the coating film will be rougher than if itis still fluid, This is due to the fact that as the film breaksaway from"the applicator roll, it is pulled "up into This is important from the i53; spherical droplets because of the forces of surface tension andcohesion. If the film is still fluid when leaving this zone, thesespheres flow together into a smooth surface. If the film is no longerfluid, the small spheres become permanently set and a rough film resultsand poor fiber coverage also.

It has been found that as the color density increases the surface sheenalso increases, and better fiber coverage results. It is highlydesirable that the color density be controlled to obtain the desiredcoating.

Mineral-adhesive coating compositions are thixotropic to a degree, andthis property is quite significant in the present method of coating.Advantages are obtained by applying the coating at high solids contents.This raises the question as to how high solids content may go with agiven mineral-adhesive combination. The answer appears to be that thesolids content should not exceed the point where the coating begins toshow a'pronounced degree of dilatancy at high rates of flow. This pointcan be determined for any given composition by plotting rates of flowagainst force or viscosities against rates of flow.

The doctor roll is driven through suitable medium such as a variablespeed Reeves drive so that the speed ratio between it and the applicatorroll can be varied. The variable speed gives a great flexibility incontrol over the coating weight. For example, with a starch clay coatingof 65% total solids and with a constant pressure on the doctor rollthere is obtained a variation in coating weight from 12 pounds at aspeed ratio of 1111.2 to four pounds at 1:192 at a constant machinespeed.

It has been found that when colors of high solids are used a very thinfilm of coating passes under the doctor blade and dries before coming incontact with the applicator roll. This causes markings in the coatingfilm on the applicator roll which will be transferred to the paper. Thisdifficulty can be overcome by placing the doctor blade as near theapplicator roll nip as possible and keeping the doctor roll wet.

It has been found that in order to obtain the smoothest coating film itis necessary to keep the nip between the doctor roll and the applicatorroll flooded with a large excess of color. A number of ways may be usedto accomplish this and one way is disclosed in the accompanyingdrawings. Air bubbles may accumulate in the color fountain between thedoctor roll and the applicator roll. These bubbles may pass through thenip and cause skipped areas in the coating and the elimination of thesebubbles is essential to a satisfactory coating.

If drier wrinkles are permitted to form there will appear in thefinished sheet parallel lines of variable ink absorption. Thisdifiiculty can be overcome by the use of a felt that comes into contactwith the coated surface of the web of paper as soon as possible afterthe coating has been applied without marring the coated surface. Thecoated paper web wrinkles on the drier surface due to uneven drying andthe use of a felt which holds the coated web in close contact with thedrier drum surface causes a redistribution and equalizing of themoisture in the the drying coated web and substantially uniform dryingand this results in the elimination of drier wrinkles. The use of a feltalso permits higher drying rates with reduced drying temperature.

Figure l discloses diagrammatic illustration of an apparatus forcarrying out the new method of coating, and

Figure 2 is a side view in elevation of a coating fountain and a coatingbacking roll.

Referring to the drawing in detail where l0 indicates a web of papercoming from a suitable source of supply (not shown). The web passes overroll ll and roll I! and then into contact with the large diameter heateddrier drum I 8. The paper is coated preferably adjacent the point ofcontact with the surface of the drier surface. The paper web travelsover a major portion of the drier drum surface. A suitable coating unitindicated at 9 applies the coating to the fast moving web of paper. Theweb leaves the surface of the drier drum at roll 23.

A felt 22 contacts the coated surface of the paper for a predeterminedarc of the surface drier drum and the arc of contact is varied as thespeed of the drier drum varies, the type of coating employed, thetemperature of the drier drum, etc. The felt leaves the surface of thecoated paper at 2! and the paper and felt 22 travel together from roll23 over roll 24 to roll 25. The felt then passes over rolls 21, 28, and29 and 35! back to the surface of the drier drum.

The web of paper with one side coated passes over roll l2 and then overroll [3. The web contacts the surface of the heated drier drum |8a andtravels over the major surface of the drier drum. The web is coated by asuitably constructed coating unit indicated generally at [9a adjacentthe point where the web first contacts the drier drum. It is to beunderstood that the coating may be applied just prior to or at the timethe web contacts the surface of the drier drum. The coated web leavesthe drum at roll 20a and then passes over rolls 23a, 26, 3|, 32, and 33.The paper after passing over these last mentioned rols then may besubjected to other processing or may be wound into rolls on a suitablyconstructed wind up apparatus (not shown). The felt 22a leaves the rollsurface at roll 21a and passes over rolls 23a, 26, 3|, 28a, 29a and 35awhere it comes into contact with the coated side of the paper web on thedrier drum surface. The amount of contact of the felt with the coatedpaper on the drier drum is varied by adjusting the position of rolls 28aand 29a, and 30a.

In Figure 2 of the drawings is shown a coating unit and a heated drierdrum backing roll. A resilient surfaced applicator roll l8 contacts thesurface of the web of paper on the drier drum and the speed of theapplicator roll is preferably about the speed of travel of the paperweb. In the fountain 37 is mounted a rotatable doctor roll 35 whichcontrols the amount and the smoothness of the coating on the applicatorroll. A flexible seal 36 contacts the surface of the applicator rolladjacent the nip between the doctor roll 35 and the applicator roll 34.

Secured to the machine frame so at 39 is applicator roll supportingframe 38 and hingedly or pivotally connected to frame 38 is doctor rollsupport 42, at the upper end of member :%2 is secured member 43 andmeans :54 for varying and controlling the pressure applied by theapplicator roll 34. For varying and controlling the pressure betweenapplicator roll 34 and the heated drier drum surface is member 45 andpressure varying means 46. The member 45 is connected to member 38 atone end and to machine frame member 47 at the other end.

The coating material is supplied to container 49 and withdrawn from thecontainer through pipe. fiorby pump 52 and: is .forced' into. the:ileum.

tam;- er through pipe 56. The pipe- 56.. enters" fountain preferablynearthe topof the fountain; Aw surplus of coating material; is.maintained atthe nip. between the rolls. 35 and 34.. A surplus ofcoating material. fiowsover the. top of thezback of the fountain intocontainer 49 and: fiowssover the incline plate 48. positioned in thecontainer;

inclined plate aids in eliminating entrained: air inithe coatingmaterials.

To clean the: surface. of the: doctor roll= a resilient. doctor blade54: contacts the. surface: of the. doctor roll. 35. provided so that thepressure between the doctor blade and the doctor roll can be controlled.The

doctor blade is carried by member 53 which is attached to. doctor rollsupporting frame 4'2. Some of the factors which determine the amountxof;coating applied to a traveling web: of paper:

Variable machine speed Constants:

Color solids Doctor roll pressure; Doctor r011 speedratio. Doctor r011speed;

Ratio 4:1

Machine Coating Speed, Wgt, Lbs. F. P. M. per Ream Variable doctor rollspeediratio Constants:

Coating solids Doctor roll pressure Machine speed A pressure varying:means: 551

Variable color solids;

Constants:

Machine speed Doctor roll pressure Doctor r011 speed ratio Per CentCoating Wgt. SolidsIn Color Lbs. per Ream The present process hasbeenccarri'ed out very" successfully Wherethe applicator roll is. backedby a drier rollzoi' 8 feet indiameter but it is to. be understood: thatthe drier roll employed may vary."

considerably in diameter from that indicated;

The effect of the nip pressure betweenthedrier:

roll and the. applicatorroll" controls the ty-peof coating applied tothe. moving paper web: The 4 density of: the coating: material may v-aryoven a considerable 1 range, but. highly successful coating Ratiobetween Surface speed i l .Sum of Doctor Roll and ztigrar g lalhcatorroll and doc DOCtOI RollSpeed F.P..M. Applicator .Ronspeeds Coatmg Wgt.1n lbs/ream 625 F. P. M. 492 F. M. P. 625 F. M P. 492 F. P. M. 625 F. PM 27 tooth gear 36 tooth gear 27 tooth gear 36 tooth gear 27 tooth gearC has been obtained where the solids contents of Variable doctor Topressure the coatmg falls Within the range of about 61% Constants:

Machine speed Doctor roll speed ratio Coating solids to about 72% solidsand the paper web was traveling at speed of about 800 feet per minute toabout i 1290 feet per minute.

One important advantage resulting from the high speeds at which webs canbe coated with a coating material of high solids content is that paperwebs may be coated with a uniform, smooth and selected weight of coatingmaterial as they are delivered from a paper making machine substantiallyas part of the paper making operation or as a separate operation it ispreferred that the coating color be applied to the paper when sub- Theadditions of stantially dry, that is, having a moisture content fromabout 2% to about 10%.

It will be understood that the term color as used in this specificationand claims, includes any of the coating materials usually employed forcoating purposes.

Various changes from the detailed description and specific steps of themethod will occur to those skilled in the art Without departing from thespirit ofthe invention or the scope of the appended claims.

What we claim:

1. A method of coating a continuously advancing web of paper whichcomprises, depositing a controlled and a Substantially uniform amount ofmineral pigment coating material of high solids contents upon and evenlyspread across the width of a rotating resilient surface, rotating saidsurface in contact with a supported surface of a web of paper, and inthe same direction of movement with said Web, and at a speedsubstantially the same as the translatory speed of the said web to applya uniform coating to said web surface, and subsequently contacting thecoated surface of said web with a movin porous surface after the coatingmaterial has received a preliminary set.

, 2. A method of coating a traveling web of paper,

which'comprises applying a coating composition comprising mineralpigments to a cylindrical surface having resilient properties, removinga portion of said coat ng so applied by a rotatable surface whereby saidcylindrical surface has thereon a uniform amount of coating compositionwhich is less than the amount initially present on said surface, thencontacting said cylindrical surface with a moving web of paper, drawingthe web over the major portion of a heated drier drum surface, andcontacting the coated surface of the web of paper while on the driedsurface with a moving felt surface after the coating has received apreliminary set.

3. A method of applying mineral coating of a solids content of about 61%to about 72% to a fast moving Web of paper having a moisture content ofabout 2% to about 10%, comprising applying the mineral coating to arotatable resilient surface, transferring the coating on the resilientsurface to the Web of paper, carrying the web of paper in contact with aheated supporting surface for a portion of the surface of the saidsupportingsurface, and redistributing the moisture content of the coatedweb of paper while on the said supporting surface by means of a movingfelt after the coating has received 'a preliminary set.

4. The method of applying mineral coating of high solids contents to afast moving web of paper, which comprises applying a wet mineral coatingin a smooth film to a face of the paper Web, moving the web forwardly ona round heated supporting surface, and bringing a moving felt intocontact with the coated surface of the web on said supporting surfaceafter the coating material has received a preliminary set.

5. A method of coating a fast moving web of paper comprising, applyingmineral coating material of solids content in excess of sixty per centto a web of paper, carrying the web of paper into contact with a heatedcylindrical surface for a major portion of said surface and contactingthe coated surface of the paper during its travel over a portion of acylindrical drying surface with a moving porous surface forredistributing the moisture contents of the coated web after the coatinghas received a prel'minary set.

WILLIAM H. BARRETT. BERNARD K. ASDELL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,866,689 Wilshire July 12, 19321,933,963 Bradner Nov. 7, 1933 2,133,933 Daley Oct. 18, 1938 2,204,600Kabela et al June 18, 1940 2,267,470 Kabela et al so. 23, 1941 2,293,690Harrigan Aug. 18, 1942 2,304,819 Grupe Dec. 15, 1942 2,313,497 AdrianMar. 9, 1943 2,331,922 Montgomery Oct. 19, 1943 2,337,013 Bradner et a1Dec. 14, 1943 FOREIGN PATENTS Number Country Date 320,477 Great Britain1929

3. A METHOD OF APPLYING MINERAL COATING OF A SOLIDS CONTENT OF ABOUT 61%TO ABOUT 72% TO A FAST MOVING WEB OF PAPER HAVING A MOISTURE CONTENT OFABOUT 2% TO ABOUT 10%, COMPRISING APPLYING THE MINERAL COATING TO AROTATABLE RESILIENT SURFACE, TRANSFERRING THE COATING ON THE RESILIENTSURFACE TO THE WEB OF PAPER, CARRYING THE WEB OF PAPER IN CONTACT WITH AHEATED SUPPORTING SURFACE FOR A PORTION OF THE SURFACE OF THE SAIDSUPPORTING SURFACE, AND REDISTRIBUTING THE MOISTURE CONTENT OF THECOATED WEB OF PAPER WHILE ON THE SAID SUPPORTING SURFACE BY MEANS OF AMOVING FELT AFTER THE COATING HAS RECEIVED A PRELIMINARY SET.